Multifrequency signaling is now commonplace in communication systems. It is employed in subscriber signaling, signaling between central offices, intraoffice communications, remote control of other systems, control of remote test equipment, inputting data to computer systems, and the like. Consequently, it is increasingly important that detection of valid multifrequency signals be achieved accurately and inexpensively.
Heretofore, numerous arrangements have been proposed for detecting reception of "valid" multifrequency signals. In prior multifrequency receivers automatic gain control circuits are used to insure the signal supplied to the multifrequency detector is adjusted to a prescribed level. In such arrangements, the automatic gain control circuits are used to insure the signal supplied to the multifrequency detector is adjusted to a prescribed level. In such arrangements, the automatic gain control circuit locks onto the strongest tone in the incoming multifrequency signal and adjusts that tone to a prescribed amplitude level. Consequently, all other tones comprising the incoming signal are adjusted by the same gain and they are not all adjusted to the same amplitude level as the strongest tone. Therefore, to assure confidence in detecting "valid" multifrequency signals and to guard against detecting out-of-band signals, it became the practice to first determine whether one or more tones which exceed a first prescribed amplitude level are present, for example, greater than -5 dB relative to a single tone center of band output from the receiver automatic gain control for the frequency of the particular multifrequency tone and, then, determine whether two and only two tones are present which have a magnitude greater than a second prescribed threshold level, for example, greater than -10 dB relative to the single tone center of band output from the receiver automatic gain control at the frequency of that tone. The -5 dB threshold corresponds to a tone well within the passband of a corresponding filter. The -10 dB threshold corresponds to a tone having 6 dB of twist relative to the other received tone and being at the edge of the filter band for that particular tone. If two and only two tones are present for a prescribed interval they are considered to represent a valid multifrequency command.
Prior multifrequency receiver arrangements tended to approximate the above-stated multifrequency reception objectives. More recently, however, U.S. Pat. No. 4,091,243 issued to A. Mizrahi et al., on May 23, 1978 discloses an arrangement employing a control circuit in conjunction with a controllably adjustable or settable reference threshold level and a plurality of threshold detectors to monitor automatic gain controlled received signal outputs from a plurality of bandpass filters. The reference threshold level supplied to the detectors is controllably adjusted in order to effect the prior telecommunications multifrequency reception objectives stated above. Specifically, upon initialization of the multifrequency receiver, a reference threshold level supplied to the threshold detectors is first set by the control circuit to a first prescribed threshold level, for example, the -5 dB level. Upon detection of at least one tone at one of the desired frequencies having a magnitude which exceeds the first threshold level the control circuit sets the reference threshold level supplied to the detectors to a second prescribed threshold level, for example, the -10 dB threshold level. Thereafter, a routine is effected to determine whether two and only two tones which exceed the -10 dB threshold have been received and have been present for a prescribed interval.
One problem with the prior multifrequency receivers which employ automatic gain control circuits is that tones having magnitudes below the second threshold level of -10 dB are considered invalid. Consequently, tones having greater than 6 dB of twist are rejected and receiver sensitivity is therefore limited. As indicated above, this sensitivity limitation was necessary to guard against erroneous detection of out-of-band signals as valid multifrequency signals. Additionally, the prior arrangements have had to determine whether the tones exceeded first and second threshold levels thereby having to make a determination of tones being present at both these levels. The use of multiple threshold levels is inefficient and also tends to limit sensitivity of the receiver.